1. Field of the Invention
The present invention relates to an image processing method for a display device. More particularly, the present invention relates to an image compression method, an image decompression method, and a motion picture detection method for a thin film transistor liquid crystal display device (TFT LCD).
2. Description of Related Art
In the past few years, LCD devices have been widely used to replace traditional cathode-ray tube (CRT) display devices. Presently, due to the development and progress of TFT technology, using a TFT as an image pixel of an LCD has become very popular. FIG. 1 illustrates a normal process for processing an image for a TFT LCD. With reference to FIG. 1, input images from an image source 100 are transmitted through a transmission channel 104, and then the images are processed, which is represented by an image processing 108 rectangle in FIG. 1. A frame memory 112 is used to store the images and the images are later retrieved therefrom to continue processing and then be displayed on a TFT LCD 116.
However, the response time of liquid crystal molecules of an LCD for displaying motion pictures is generally slow. To improve (shorten) the response time of TFT LCD devices, image pixels of a motion picture are commonly processed with overdrive technology. In general, motion pictures are displayed at a rate of about a time frame per 16 ms (millisecond). When motion pictures are continuously displayed, the image pixel information of the previous time frame should usually be stored and compared with that of the current time frame in order to determine the scale of overdrive, and this also requires a frame memory buffer to support the storage and retrieval of image pixels.
However, storing all image pixels in a complete time frame requires a large frame memory buffer, particularly for a large TFT LCD panel with high resolution. Also, the concurrent storage and retrieval of image pixels utilizing the frame memory requires a very high bandwidth bus to access the frame memory, and this makes it difficult to implement the bus interface and induces significantly high electromagnetic interference (EMI) in the TFT LCD panel.
In order to reduce the size of the frame memory and solve the problem of high EMI, image compression methods, such as discrete cosine transform (DCT) algorithm or hierarchical vector quantization method, are often employed. Image compression based on DCT algorithm or vector quantization method may create some artifacts, which degrade the video pictures with artificial text or graphical patterns, and thus still require image compression with higher resolution for fine details.
In another respect, the overdrive for response time improvement should be activated only when the given images are motion pictures. Because the image source may itself be noisy or the images may be transmitted through an unreliable transmission channel easily coupled with noise, still pictures may also be treated as motion pictures, so that the overdrive intended to improve the response time for motion pictures may lead to noise amplification for still images with unpleasant visual effects.